(1) Field of the Invention
The present invention relates to a metal vapor discharge lamp such as a metal halide lamp, and in particular, relates to a metal vapor discharge lamp having a nitrogen gas in its outer tube.
(2) Related Art
A general structure of a metal vapor discharge lamp is such that an arc tube having mercury, a rare gas, and a luminous metal, is placed in an outer tube already having a nitrogen gas therein, and that electrodes of the arc tube and a base of the outer tube are electrically connected by wiring members. A nitrogen gas is used as an inert filling gas, and also plays a role of maintaining the temperature of the arc tube to an adequate level during the lamp illumination. The wiring members are composed of a power supply line, a stem line, and the like, that are placed inside the outer tube. The wiring members are used to support the arc tube from inside the outer tube, and also used to hold a sleeve surrounding the arc tube, when necessary. Such wiring members are required to have a high-heat resistance property against the high temperature resulting during the lamp illumination. A conventional example of the wiring members is a metal member mainly composed of iron (e.g. iron, nickel-plated iron, and an iron-nickel alloy).
Meanwhile, the inventors of the present invention have observed that a conventional metal vapor discharge lamp has a problem in that a nitrogen gas pressure inside the outer tube will decrease as the lamp is used over a long period of time.
As a nitrogen gas pressure decreases as stated, a nitrogen gas ceases to work as a filling gas for the outer tube. Accordingly, the temperature in the arc tube will not be adjusted properly during the lamp illumination. As a result, it becomes very likely to cause a problem of changing the life property which is attributable to the increase in the vapor pressure of the arc tube. Furthermore, the internal pressure of the outer tube will decrease as the decrease in nitrogen gas. Accordingly, at such times as applying a high pressure pulse to the lamp, and non-illumination time attributable to leakage from the arc tube at the end of life of the lamp, it becomes possible that an arc discharge or a glow discharge is generated between the wires inside the outer tube (i.e. between opposite polarity electrodes), thereby causing an electrical breakdown.
This seriously damages the illuminating performance of lamps, and is considered a problem to be solved immediately.